1. Field of the Invention
The invention relates to a reproduction technique of an optical disc drive.
2. Description of Related Art
The invention is applicable not only to Blu-ray Disc (BD) drives but also to other types of optical disc apparatuses. Nevertheless, the descriptions that follow are based on the BD drives and the terms that are used in the following descriptions are basically the same as those in the field of the BD drives.
Many of the existing optical disc apparatuses, such as BD drives adopt the high-frequency superimposing method to reduce noise that is produced by the laser diode used as its light source. This technique is disclosed in Non-Patent Document 1 and has been known to those skilled in the art. So, the descriptions of the technique are limited to only a part thereof that necessary for the descriptions that follow. No further details will be described.
In the optical system of an optical disc drive, if a part of the laser beams that are reflected by the surface of the disc enters the oscillating laser diode, such entry destabilizes the oscillation of the laser diode, and thus produces a significant amount of laser noise. The high-frequency superimposing method is used to avoid the production of such laser noise. The name “high-frequency superimposing” method comes from the fact that the laser diode emits pulsed laser light by superimposing a high-frequency signal over the driving signal of the laser diode. FIG. 2 illustrates the waveform of the emitted light. Light emission and light extinction are repetitively alternated. Note that the ratio (duty) of each light-emitting period to each interval between two adjacent laser pulses (modulation frequency) is a parameter that is to be adjusted so as to minimize the laser noise. Specifically, the frequency and the duty are selected so that the laser pulse that is reflected by the surface of the disc cannot enter the oscillating laser diode.
If the pulsed laser beam is focused on the recording film of the optical disc, the intensity of the reflected laser beam varies depending upon whether the position irradiated with the laser beam is a mark or a space. Accordingly, the amplitudes of the laser pulses are modulated. If there is no bandwidth limitation imposed by the photodiode for reproduction or by the current to voltage amplifier, the read signal waveform will look like as shown in FIG. 3. Hereafter, the signal made of such a read pulse string will be referred to as the pulsed read signal. The dashed line in FIG. 3 represents the read signal waveform obtained when the laser is made to oscillate continuously with an output level that is the same as the level of the peak of the laser pulse at the time of high-frequency superimposition. To put it differently, the upper-side envelope of the pulsed read signal has a shape of the read waveform caused by the continuous light. Thus, a desired read waveform can be obtained by an envelope detection, that is, by passing the pulsed read signal through a low-pass filter having a cut-off frequency sufficiently lower than the frequency of the high-frequency current that is superimposed on the pulsed read signal. The existing optical disc apparatuses achieve the desired read waveform by means of a bandwidth limitation that is imposed by a system including a photodetector and a current to voltage amplifier as well as by an analog equalizer.
Pulsing read signal is a sort of amplitude modulation. Thus, not only the line-like spectrum of the superimposed high-frequency signal, but also the components of the modulated read signal are observed near the line-like spectrum. For this reason, in the following descriptions of this disclosure, the superimposed high-frequency signal will be referred simply to as the “carrier.”
A possible, standard frequency for carrier is, for example, 400 MHz in the case of BD drives. The frequency depends solely on the optical path length of the read optical system, so that there seem to be no big differences among different apparatuses.
FIG. 4 illustrates an exemplar spectrum of a pulsed read signal. The dashed line in FIG. 4 represents, schematically, how the bandwidth limitation is imposed by the system including the photodetector and the current to voltage amplifier as well as by the analog equalizer. As shown in FIG. 4, the conversion of a pulsed read signal to a continuous signal according to the conventional method can be accomplished by totally attenuating the harmonic components. Accordingly, the amplitude of the obtained read signal becomes smaller. The ratio of the amplitude thus obtained to the amplitude of the pulsed read signal is approximately equal to the pulse duty.
A technique known as the multi-tone demodulation (MTD) is a technique to prevent such decline of the SNR. Detailed descriptions of this technique are disclosed in Patent Document 1, and Non-Patent document 2 also describes the technique.
[Patent Document 1] Japanese Patent Application Publication No. 2007-73147
[Non-Patent Document 1] Akira Arimoto et al., “Kosyuha-denryu cyojyohou ni yoru handotai-reza tosai video-disuku-pureiya no reza-noizu teigen-ka (Reduction in Laser Noise of Video Disc Player Equipped with Semiconductor Laser by High-Frequency Current Superimposing Method)”, Kogaku (Optics), vol. 14, no. 5, pp. 377-383.
[Non-Patent Document 2] Frank Op't Eynde and Willy Sansen, Analog Interfaces for Digital Signal Processing Systems, Kluwer Academic Publishers, 1993, Boston/Dordrecht/London, pp. 91-92.
[Non-Patent Document 3] Atsushi Kikukawa and Hiroyuki Minemura, “Novel HF-pulse read signal converter for increasing read signal SNR”, Digest of International Symposium on Optical Memory 2007, pp. 302-303.